Transformers & motors
Motor Current Estimator
Estimated motor current starts by converting mechanical output to electrical input using efficiency. Current then depends on voltage, phase, and power factor.
Estimate motor input current from mechanical output, voltage, phase, efficiency, and power factor.
Calculated result
Estimated motor current
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
Use the actual nameplate and governing design rules for installation work.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Pinput = Pmechanical/efficiency; I = Pinput/(phase factor × V × PF).
- P
- Real or mechanical power, as identified by the calculator.
- S
- Apparent power in VA or kVA.
- V
- Winding, line, or load voltage.
- I
- Winding, line, or estimated input current.
- PF
- Entered power factor where real and apparent power differ.
- η
- Entered conversion or motor efficiency.
1 mechanical horsepower is 745.699872 W; phase factor is 1 or √3.
Worked example
| Input | Value |
|---|---|
| Power unit | Horsepower |
| Mechanical output | 5 hp / kW |
| Voltage | 480 V |
| Phase | Three-phase AC |
A 5 hp, 480 V, three-phase motor at 90% efficiency and PF 0.85 is estimated at about 5.87 A.
Reference table
| Apparent rating | 240 V single-phase | 480 V three-phase |
|---|---|---|
| 1 kVA | 4.17 A at 240 V 1φ | 1.20 A at 480 V 3φ |
| 5 kVA | 20.83 A at 240 V 1φ | 6.01 A at 480 V 3φ |
| 10 kVA | 41.67 A at 240 V 1φ | 12.03 A at 480 V 3φ |
| 25 kVA | 104.17 A at 240 V 1φ | 30.07 A at 480 V 3φ |
Frequently asked questions
Why can calculated motor current differ from nameplate current?
The estimate depends on entered output power, efficiency, power factor, voltage, and phase; the nameplate reflects the actual motor design and rating method.
Does this include motor starting current?
No. It estimates steady-state input current and does not model locked-rotor or acceleration current.
Assumptions and limitations
- Not NEC full-load current and not a nameplate replacement.
- No starting current, service factor, derating, protection, or conductor sizing.
Method and sources
Review the electrical formulas, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .